Lunar Excavation Payload
The Lunar Excavation Project is developing a prototype payload for the Polaris lunar rover designed to collect lunar regolith in mass quantities to send back to Earth. In the past several weeks, design progressed from concept to technical models (Figure 1). Recently, the team made major revisions and modifications to the assembly to improve performance, reliability, and manufacturability.
The dumpbed design was significantly modified and refined. With new information on the critical tipping angle at which regolith can be completely dumped, the slope of the back wall of the dumpbed was steepened (Figure 2a). Doing so increased the carrying capacity by approximately 15 kilograms. In addition, the outer profile was changed to that of a cylinder to better conform to the trench dug by the digging “bucketwheel” during excavation and reduce dragging in the regolith (Figure 2b). Moreover, the portion of the dumpbed set into the bucketwheel now conforms to the inner circle of the bucketwheel to increase flow rate and overall capacity, and the height of the lip was reduced to increase collection efficiency. The motor driving the bucketwheel must be mounted on the inside of the dumpbed, so a sheet metal cap was added to shield the motor from falling regolith. The rims around the holes onto which the pivoting motor and torque tube attach were thickened to add support, and metal braces were added below the holes to better distribute the loads and reduce warping. The overall thickness of the walls also decreased from 5 millimeters to 2 millimeters, reducing the total weight of the dumpbed itself significantly. The front of the dumpbed onto which the bucketwheel motor mounts, however, remains 5 millimeters thick to better support loads from the bucketwheel. Gussets were added to improve the stiffness and reduce the deflection under loads (Figure 3a, 3b).
These modifications resulted in a simplified dumpbed which may now be easily manufactured by means of making a cutout in sheet metal, bending it at corners, and welding it together. Overall, the dumpbed now has a capacity of approximately 74 kilograms (without using agitation to flatten the conical shape of deposition), has a weight of 9 kilograms, and can be manufactured from two bent and welded pieces of 2-millimeter-thick sheet metal and one 5-millimeter-thick plate.
The wall thickness of the actuator arms and torque tubes (one at the arm base and one rotating the bucketwheel-dumpbed assembly) decreased to 3 millimeters, and stress analysis indicated that they are still sufficiently strong to handle their respective intended loads. To make the design as compact as possible, the team selected the Micromo 4490 BS series motor which, in combination with a 60:1 ratio right angle planetary gearbox and a 160:1 harmonic drive reduction, has a profile that can fit within the carbon fiber rectangular tubes of the Polaris chassis and can be fixed to the end of the actuator arm without occupying much payload space. Each motor (with gearboxes) can output 814 Newton-meters of torque, which can comfortably lift and rotate a fully-loaded dumpbed. In addition, each motor will have a Micromo MBZ Series motor brake, mounted on the opposite side of the motors themselves, to hold the assembly in place during excavation and transportation. An IE1024 encoder affixed to the other side of the brakes will provide actuator feedback.
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The new, improved designs are more robust and easier to manufacture and still meet all previous requirements. The module can extend beyond 30 centimeters below the lunar surface by making several passes over the same area, each one cutting 5 centimeters deeper into the trench (Figure 4). By tilting the mechanism 142 degrees, the dumpbed can dump a full load of approximately 74 kilograms of material behind the rear wheels and at a height greater than the 50 centimeter requirement (Figure 5). The entire assembly will weigh approximately 25 kilograms.
The model also has clearances to allow for positioning errors and displacement due to unexpected loads (for example, a rock wedging itself against the bucketwheel). A gap that is at all times at least 4.5 centimeters wide separates the bucketwheel from the Polaris crossbeam support, ensuring they will not collide when the bucketwheel is parallel to it (Figure 6a). In a scenario where the bucketwheel is not parallel to the crossbeam, an angular discrepancy of up to 8.38 degrees can be tolerated before a collision occurs (Figure 6b).